Optical fibre amplifiers are used to amplify either continuous or pulsed optical signals. There are two main techniques for amplifying ultrashort light pulses, such as femtosecond pulses, using optical fibre amplifiers.
The first technique, called fibre chirped-pulse amplification, uses a combination of three subcomponents, namely a stretcher, an optical fibre amplifier and a compressor. The stretcher is a dispersive optical element which introduces a spectral chirp in the short pulses to be amplified so that the frequency content of each pulse is spread over time. As a result, the pulse duration is increased, and the peak power of the pulse is reduced.
The chirped pulses are then injected in a standard optical fibre amplifier. Pulse amplification takes place with low nonlinear effects, as the peak power is reduced. After amplification, a dispersive compressor is used to bring the frequency components of the amplified pulses back in phase, causing the pulse to retrieve its original (short) duration.
A second and more recent technique is called parabolic pulse amplification (see Fermann, “Self-Similar Propagation and Amplification of Parabolic Pulses in Optical Fibers”, Physical Review Letters 84 #26, p6010 (26 Jun. 2000)). The origin of this technique is the observation of an asymptotic solution to the NonLinear Schrödinger Equation (NLSE) for short pulses guided in an optical fibre showing gain and normal dispersion. The shape of the pulse corresponding to this asymptotic solution is a parabola, hence the name parabolic pulse amplification. A pulse being amplified in the parabolic regime gets an increasingly broader spectrum and a linear chirp together with a higher energy as it propagates in the optical waveguide.
Parabolic pulse amplification is typically used in amplification of femtosecond pulses produced by a femtosecond laser oscillator, such as a mode-locked fibre laser. A femtosecond fibre laser oscillator usually has a pulse repetition frequency between 5 and 100 MHz. On the other hand, for some applications such as material processing, a pulse repetition frequency of about 100 kHz is desirable. In parabolic pulse amplification, an adequate balance of dispersion, nonlinearity and gain has to be present throughout the length of the amplifier for the parabolic asymptotic solution to be reached. Reducing the pulse repetition frequency of the input signal, and consequently reducing the average power of the input signal, results in a small signal gain regime in the front end of the amplifier. This results in an unbalanced gain and to improper conditions for parabolic pulse amplification.